Heat oven to 190C/170C fan/gas 5. Melt the butter in a pan, stir in the flour and cook for 2 mins. Add the onion and garlic and cook for 3-5 mins until soft. Add the mascarpone, then slowly add the spinach liquid, stirring until smooth. Cook for 4-5 mins until the sauce thickens.

Stir in the cheddar, jalapeños, celery salt, paprika and spinach. Transfer to a baking dish and bake for 40 mins until golden.